1. Field of the Invention
The present invention relates to aircraft auxiliary power units and diagnostic test equipment for them.
2. Description of the Related Art
An auxiliary power unit (APU) provides electrical and pneumatic power to an aircraft when its main engines are not operational, such as when the aircraft is parked. An APU typically includes a jet turbine engine that drives an electrical generator and other equipment, an electronic control unit (ECU), and cockpit-mounted controls. In APUs manufactured by Hamilton Sundstrand, the ECU is also referred to as a full authority digital electronic control (FADEC).
If an APU fails, an aircraft maintenance technician may be called upon to diagnose and correct the problem. Typically, the ECU measures and captures in a non-volatile memory a limited amount of data relating to various APU operating parameters as they were at the time the APU was started and at the time the APU failed. It may capture such data relating to not one but several recent startups and/or failures, e.g., the 12 most recent failures. From this data, the ECU may provide the technician with some limited diagnostic information. For example, a typical ECU has a one-line alphanumeric display on which it displays a fault code such as "HOT" to indicate a High Oil Temperature condition or "ACCL" to indicate a failure of the APU to accelerate to operating speed upon startup. The ECU may include a simple user interface such as a single button that the technician can press to scroll through the fault codes for recent failures. The ECU may record and indicate more than one fault code relating to a failure. For example, scrolling through the records may indicate both a "HOT" fault code and a "ACCL" fault code associated with the most recent failure.
APU fault codes typically represent only a fraction of the potential faults, and typically do not provide the technician with sufficient information to diagnose the cause of the failure. For example, the fault code "ACFT" may indicate only that the fault relates to the aircraft/APU interface. The intent of the displayed information is only to provide a starting point for diagnosis. Technicians must refer to the APU manufacturer's maintenance manual to determine the significance of the code and the possible causes of the problem. There may, for example, be at least six different reasons why the ECU displays the "ACFT" fault code. The manual may instruct the technician to observe other APU conditions or perform further tests to determine which of these reasons is the applicable one.
Systems have been developed to aid a technician in determining which of the possible reasons for failure is associated with a particular fault code. Such a system may include a laptop computer that the technician can couple to the ECU. The computer downloads from the ECU the fault codes as well as additional diagnostic codes that the ECU stores in association with each failure. These additional codes may indicate the state of various APU subsystems at the time of failure. Nevertheless, the ECU stores these additional codes in binary format only, and does not provide any type of mnemonic translation that would be meaningful to a technician. The primary function of the prior laptop-based systems has been simply to read the internal binary codes, translate them into verbiage, and display them.
Even with the aid of a laptop-based system that displays the internal fault codes in the form of verbiage meaningful to a technician, the task of correctly diagnosing and correcting a problem remains daunting due to a number of complicating factors. One such factor is that, of the internal codes that are displayed, only some of them may actually relate to the cause of the problem, while the others relate to effects of the problem. It is difficult for a technician, even with the aid of a maintenance manual, to determine which of the displayed error codes provide meaningful clues for diagnosing the problem and which do not. Further complicating diagnosis, it may not be evident to the technician which of the different fault codes indicate the more severe problems and, as a result, a technician may spend time needlessly diagnosing less severe problems, which may even be effects rather than the causes of the failure, before more severe problems, which are generally more likely to be the causes of the failure.
The data that the ECU records may include more data than that noted above, including, in certain cases, data relating to engine operating parameters such as speed and temperature. This data has been intended for use only by the APU manufacturer's service personnel and has not been usable by maintenance technicians in the field. The APU manufacturer may on occasion retrieve ECUs from aircraft in the field and, by downloading and analyzing this additional data, assess the manner in which its APUs are being used or investigate engineering design issues.
Prior APU diagnostic systems have not advanced considerably beyond the display of cryptic and imprecise fault codes. It would be desirable to provide a troubleshooting system that not only precisely diagnoses APU faults but also recommends corrective action. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.